Stents are endovascular prostheses (endoprostheses) or implants, which may be used for treatment of stenoses (vasoconstrictions). They have a base body in the form of a hollow cylindrical or tubular basic mesh, which is open at both of the longitudinal ends of the tubes. The tubular basic mesh of such an endoprosthesis is inserted into the blood vessel to be treated and serves to support the vessel.
Such stents or other endoprostheses as well as devices which can be used in aircraft engineering often have metallic materials in their base body. The metallic materials may form a biodegradable material which may also contain polymeric biodegradable materials.
“Biodegradation” is understood to refer to hydrolytic, enzymatic and other metabolic degradation processes in a living body, caused mainly by body fluids coming in contact with the endoprosthesis and leading to gradual dissolution of at least large portions of the endoprosthesis. The term “biocorrosion” is often used as synonymous with the term biodegradation. The term “bioresorption” includes the subsequent resorption of degradation products by the living body.
Materials that are suitable for the basic body of biodegradable implants may comprise multiple materials. Examples of suitable polymer compounds include polymers from the group including cellulose, collagen, albumin, casein, polysaccharides (PSAC), polylactide (PLA), poly-L-lactide (PLLA), polyglycol (PGA), poly-D,L-lactide-co-glycolide (PDLLA-PGA), polyhydroxybutyric acid (PHB), polyhydroxyvaleric acid (PHV), polyalkyl carbonates, polyorthoesters, polyethylene terephthalate (PET), polymalonic acid (PML), polyanhydrides, polyphosphazenes, polyamino acids and their copolymers as well as hyaluronic acid. Depending on the desired properties, the polymers may be used in pure form, in derivatized form, in the form of blends or copolymers. Metallic biodegradable materials are based on alloys of magnesium, iron, zinc and/or tungsten.
The present invention relates to endoprostheses or other devices having a base body whose material comprises a metallic material. In addition to the aforementioned biodegradable materials, other metallic materials may also be considered.
The position of a stent or other devices is often determined by means of imaging methods, e.g., by means of an X-ray device. Because of the low atomic number and low density of the biodegradable material, magnesium and its alloys, the radiopacity of the medical implants produced from magnesium is very low. To overcome this disadvantage, it is known that medical devices may be furnished with function elements having a different material composition in comparison with the material of the base body in at least a portion of their volume. These so-called markers or function elements contain in particular a material that absorbs X-rays and/or other electromagnetic radiation to a greater extent (hereinafter referred to as radiopaque and/or radiologically opaque material) than does the material of the base body.
The document U.S. Pat. No. 6,355.058 B1 describes a stent in which radiopaque markers are enclosed as particles in a polymer binder. The binder is distributed (dispersed) on the surface of the stent. Such a distribution of radiopaque particles usually does not result in a sufficient density of these materials, so the radiopacity is too low for many applications.
The document U.S. Pat. No. 6,293,966 B1 discloses a stent with radiopaque marker elements having C-shaped elements on its distal and/or proximal ends, these C-shaped elements form an essentially spherical receptacle. Marker elements having spherical end sections are inserted into these receptacles. The spherical end sections are attached in a form-fitting manner and, if necessary, are secured by means of a weld in the receptacles formed by the C-shaped elements.
The document DE 698 36 656 T2 shows and describes a bioabsorbable marker with radiopaque constituents for use on an implantable endoprosthesis such as a stent. The bioabsorbable radiopaque markers have porous sections, for example, which are filled with radiopaque material. Furthermore, a marker having hollow, void-like and porous sections filled with radiopaque material is also described. Furthermore, the prior art discloses a marker designed as an elongated element such as a filament, which is wrapped around parts of the implantable endoprosthesis.
With regard to other applications of a device as described above, e.g., for use in aircraft, it is customary to combine different metallic materials with one another.
With stents or other devices having base bodies made of a metallic material, the arrangement of metallic function elements on the base body leads to the problem of contact corrosion occurring in the contact area between the material of the base body and the material of the function element. This leads to destruction of the device and/or to separation of the function element from the base body, so the device is no longer capable of fulfilling its function and/or can no longer be located. The devices known from the prior art as described above do not offer a solution to the problem described here.
It is known that accelerated corrosion of implants made of magnesium alloys with X-ray markers can be suppressed by complete coatings with polymer coating materials. Both biodegradable and nonbiodegradable polymers are used here. Also conceivable are approaches in which the area where the X-ray markers are located is protected from accelerated corrosion attack over only a portion of the base body, e.g., by immersing in a polymer solution. Implants coated in this way initially have a delayed degradation behavior, but whenever the polymer coating is damaged, corrosion occurs in the same way as it would with an unprotected implant. Another basic possibility is to introduce filled polymers into the recesses provided for this purpose in the implant. The filler for these polymers consists of fine particles of radiopaque elements. Through the polymeric sheathing, the actual contact area of the marker material with the magnesium is minimized and the local element binding is suppressed. However, the holding forces exerted by the magnesium and the X-ray markers are limited due to the low strength of the polymer and the differences in the modulus of elasticity of the two materials. After only a few days, this leads to a loss of strength of the composite after only a few days and also to an associated risk that the X-ray markers will be dissolved out of the implant.